Metamorphosis of Vehicles: Mechanisms and Opportunities.

The visibility of a skin condition or dermatosis led to the reasonable assumption that the direct application of a therapeutic remedy to the target tissue holds many advantages. Through centuries, the nomenclature of topical preparations has proliferated and finally been moulded into the compulsory nomenclature of official compendia. In everyday life, many terms have been added and have complicated understanding and communication among and between healthcare professionals and laypersons. A large proportion of marketed topical preparations contain significant amounts of volatile vehicle ingredients that evaporate once they are applied onto the skin, that is, the vehicle format as well as the sum of vehicle ingredients in the primary container are different from the vehicle format and the sum of vehicle ingredients on the skin. This phenomenon and the potential consequences have so far been often ignored by many healthcare professionals and laypersons. To gain a better understanding, this phenomenon has been coined as the metamorphosis of the vehicle. The metamorphosis of the vehicle describes the vehicle (a) in the primary container (primary formulation), (b) during and immediately after application onto the skin (secondary formulation) and (c) after all volatile vehicle ingredients have evaporated from the vehicle on top of the skin (tertiary or residual formulation). The secondary and tertiary formulations may offer increased delivery of cosmetic or pharmaceutical actives. This is achieved by (a) an intended post-application creation of supersaturation of actives in the secondary and tertiary formulations or by (b) physico-chemical triggers such as pH.